1. Field of the Invention
This invention relates to intervertebral disc prostheses and more particularly to intervertebral disc prostheses having rigid endplates and an elastomeric core.
2. Background Art
Low back pain is a very common pathological condition, affecting approximately 80% of the general population at some time. Although most of patients experience the painful symptoms only occasionally and recover fully, approximately 10% of these patients come to experience chronic and disabling low back pain in spite of various medical treatments.
The most common cause of chronic disabling low back pain is degeneration of one or more of the intervertebral discs that are positioned between the vertebrae of the spine and permit the various natural movements of the spinal column. Such degenerative disc disease (DDD) may become intractable to non-surgical treatment and have to be treated by surgical intervention. Spinal fusion has been a traditional and generally effective treatment method for chronic disabling low back pain that is not responding to non-operative treatments. More recently, alternative treatments involving replacement of the entire disc or its nucleus have been developed for treatment of discogenic pain.
The first generation of prostheses for replacement of degenerated intervertebral discs has generally incorporated mutually sliding surfaces of relatively hard materials to provide for the required intervertebral motion in flexion, extension, lateral bending and torsion. Although such prostheses have been found to be helpful, improvements in shock absorption and replication of the natural motion of the intact intervertebral disc have been sought.
Accordingly, subsequently developed prostheses have incorporated elastomeric members in order to provide for the required motion and shock absorption. Such prostheses typically include relatively hard endplates for contacting the endplates of adjacent vertebrae and fixing the prosthesis thereto, together with an elastomeric disc core, positioned between the hard endplates and fastened thereto.
However, in conventional designs of such intervertebral disc prostheses, the bone-contacting members, i.e., rigid endplates, typically have a and shape and size in a horizontal plane that conforms generally to the shape and size of the vertebral endplate; and the elastomeric element, positioned between the prosthesis endplates, also typically has a similar and shape and size. When such a prosthesis is subjected to stresses induced by bending of the spinal column, e.g., flexion, the elastomeric material at the periphery of the prosthesis may be compressed between the hard endplates and caused to bulge outwardly. Such deformation of the elastomeric component in repeated flexion may lead to eventual failure of the prosthesis. In some known prostheses, the outer periphery of the elastomeric core is provided with a concavity of the lateral wall to reduce the fixation stress in the peripheral region where the elastomer interfaces with the rigid, e.g., metal, endplates. However, even such a structure may be subject to eventual failure.
The present invention has been devised in view of the above background.